A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” -direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, in the range of 5 to 20 nm, in particular around 13 nm.
Such radiation is termed extreme ultra violet (EUV) or soft X-ray and possible sources include, for example, laser produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. These types of radiation require that the beam path in the apparatus be evacuated to avoid beam scatter and absorption. Because there is no known material suitable for making a refractive optical element for EUV radiation, EUV lithographic apparatus should use mirrors in the radiation (illumination) and projection systems. Even multilayer mirrors for EUV radiation have relatively low reflectivities and are highly susceptible to contamination, which may further reduce their reflectivity and hence the throughput of the apparatus. This may impose further specifications on the vacuum level to be maintained and may necessitate especially that hydrocarbon partial pressures be kept very low.
In a typical discharge plasma source, plasma is formed by an electrical discharge. The plasma may then be caused to compress so that it becomes highly ionized and reaches a very high temperature, causing the emission of EUV radiation. The material used to produce the EUV radiation is typically xenon gas or lithium vapor, although also other gases and vapors such as krypton gas or tin or water vapor may be used. However, these gases may have a relatively high absorption of radiation within the EUV range and/or may be damaging to optics further downstream of the projection beam and their presence should therefore be minimized in the remainder of the lithographic apparatus. A discharge plasma source is disclosed, for example, in U.S. Pat. No. 5,023,897 and U.S. Pat. No. 5,504,795, both of which are incorporated herein by reference.
In a laser produced plasma source a jet of, for example, (clustered) xenon may be ejected from a nozzle, for example, from an ink-jet like nozzle as droplets or thin wire. At some distance from the nozzle, the jet is irradiated with a laser pulse of a suitable wavelength for creating a plasma that subsequently will radiate EUV radiation. Other materials, such as water droplets, ice particles, lithium or tin, etc. may also be ejected from a nozzle and be used for EUV generation. In an alternative laser-produced plasma source an extended solid (or liquid) material is irradiated to create a plasma for EUV radiation. Laser produced plasma sources are, for example, disclosed in U.S. Pat. No. 5,459,771, U.S. Pat. No. 4,872,189, and U.S. Pat. No. 5,577,092, all of which are incorporated herein by reference.
During generation of EUV radiation, particles are released. These particles, hereinafter referred to as debris particles, include ions, atoms, molecules, and small droplets. These particles should be mitigated, in the sense that a possible detrimental effect of these particles on the performance and the lifetime of the lithographic apparatus and, in particular, the illumination and projection system, be minimized. European Patent Application Publication No. 1 223 468 A1, which is incorporated herein by reference, describes a lithographic apparatus that has a contaminant barrier that includes ionization means for ionizing a gas that is provided in a region through which the radiation propagates from the source to the illuminator. This gas is provided in order to be ionized.